Surface acoustic wave (SAW) sensors, initially developed in the mid 1980's can be used to measure physical, chemical, and biological parameters. See, for example, U.S. Pat. No. 4,312,228 “Methods of Detection With Surface Acoustic Wave and Aparati Therefor”, by H. Wohltjen. These devices can be operated in wired or wireless modes. When operated wirelessly, SAW sensors have the advantage over many other wireless sensor technologies of being capable of use in a completely passive operation.
Systems using SAW devices for remote identification (or “tagging”) generally include multiple passive SAW devices, each containing a unique identification code built into the device structure, and a remotely located interrogator which can generate a radio frequency (RF) interrogation signal, analyze the response reflected from the SAW device, and thereby determine the code and identify the specific device. Such SAW tagging systems have been used for access control applications, such as automotive tags for toll booth access, since the 1990's. See the publication by A. Pohl, “A review of Wireless SAW Sensors”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control”, Vol. 47, No. 2, pp. 317-332, March 2000.
Until relatively recently, SAW sensing and tagging have been regarded primarily as separate objectives. SAW tag devices were used principally for passive wireless identification purposes, as one type of radio frequency identification (RFID) device. The general goal of RFID technology has been to replace optically readable bar codes with identification devices that can be read remotely (and while covered) using RF signals. SAW sensors, by comparison, were used most extensively for detection, identification, and quantification of volatile chemical vapors. Systems using SAW sensors for gas identification relied on complex electronics to evaluate the responses produced by arrays of SAW sensors incorporating various chemically selective coatings. These applications invariably used wired configurations for the SAW sensors. In the past decade, SAW sensor researchers recognized the desirability of combining SAW sensing and SAW tagging, and a new goal of passive, remote, RF-interrogable SAW “sensor-tags” was identified. Passive remote tag sensing using SAW devices will produce a system that is capable of identifying and tracking individual sensors in an environment in which there are several sensors within the range of the interrogator by using a built-in code or ID in each sensor. These passive sensor tags will enable remote measurement of individually identifiable sensor responses, providing both sensor identification and sensor measurement information in the passively reflected RF signal.
In his 2000 review paper on wireless SAW sensing (A. Pohl, “A review of Wireless SAW Sensors”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control”, Vol. 47, No. 2, pp. 317-332 March 2000), Alfred Pohl discusses the types of SAW devices used (delay lines and resonators) and the types of multiple access communication systems used to allow individually identifiable sensors. The approaches discussed include space division multiple access (SDMA) and time division multiple access (TDMA) for delay line devices, and frequency diversity for high-Q resonators. This summary paper also discussed the two methods for employing SAW devices wirelessly in sensing applications: (1) Using SAW one-port devices that are directly affected by the measurand as the sensor; and (2) Using two-port SAW devices that have one port electrically loaded by a conventional sensor affected by the measurand.
In 2004, Robert Brocato and his research team at Sandia National Labs discussed another technique for achieving passive remote tag sensing. See the Sandia Report SAND2004-4924 by Robert Brocato. This spread spectrum approach used SAW correlators to provide a built-in code in the SAW device.
In 2004, Don Malocha at the University of Central Florida introduced another coding scheme for producing uniquely identifiable SAW sensors, Orthogonal Frequency Coding (OFC). See the publication D. C. Malocha et. al., “Orthogonal Frequency Coding for SAW Device Applications”, Proceedings of the 2004 IEEE Ultrasonics Symposium. This spread spectrum approach to device coding requires that the SAW device consist of an input transducer and multiple reflective structures (reflector chips) with frequency responses spanning the selected wideband spectrum. These reflective structures are required to have properties that meet the orthogonality conditions that define orthogonal frequency coding. These orthogonality conditions impose strict mathematical relationships between the local (or basis set) frequencies and bandwidths of the reflector chips. The orthogonality conditions defining OFC produce reflector responses that are discrete in the time domain (rectangular [rect] functions in time, each with a specific carrier frequency sinusoid) and overlapping in the frequency domain (sin(x)/x responses with the peak of each chip frequency response occurring at the first nulls of the two adjacent chip frequency responses). The in-line configurations of the OFC devices described by Malocha also place strict limitations on the maximum possible time length of adjacent reflector chips. The imposition of the mathematical orthogonality conditions defined by Malocha results in unavoidable problems with interference between chip reflections, which causes the practical implementation of codesets with multiple codes that work together to provide unique sensor identification to be difficult. In fact, the orthogonality conditions force an unacceptable degree of spectral overlap between adjacent frequency chips. In addition, the spatial limitations introduced by the single track configuration described by Malocha severely limit the realizable efficiency of the reflector chips, causing unnecessarily high chip reflection losses. The single track approach also forces the acoustic wave to pass under reflector regions of varying frequencies. This factor, taken together with the spectral overlap of the chips, results in reflective losses from the wave that degrade the overall device response and make the response strongly code dependent.
As spread spectrum approaches, both SAW correlator sensors and OFC sensors benefit from the inherent advantages of processing gain obtained by increasing the time-bandwidth product over the data bandwidth. These techniques also benefit from communication security and reliability (resistance to jamming) inherent in spread spectrum communication systems.
Prior SAW tag-sensors utilizing OFC for coding suffer from significant difficulties in achieving adequate device performance. The present invention overcomes these limitations and provides for specific advantages over the prior art.